Magnetic alloy



J. K. STANLEY May 25, 1948.

MAGNETIC ALLOY 2 sheets-sheet i Filed oct. 3o, 194e o/ a//oyguenc/veafrakn 9/0 "6.

n, e mwv Nv M :v K0 m K ZH 5.6 5 A e N mm me; m W Q e b i fw f w .m f da s e e M m n .m w m :Z I -...s. 4,. -/.v EW ,Tm M Jl w Y NM Ac TI sm MMay 25, 1948.

2 Sheets-Sheet 2 Filed Oct. 30, 1946 INVENTOR fa/nef /C San/ey.

BY C ATTORIL wlTNEssEs;

7&0.

Patented May 25, 1948 MAGNETIC ALLOY James K. Stanley,

Turtle Creek, Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application October 30,1946, Serial No. 706,583

(Cl. Z- 123) 2 Claims.

This invention relates to alloys and in particular to magnetic alloys ofthe iron-cobalt type.

Heretofore, alloys of iron and cobalt have been avaliable for use asmagnetic material in the manufacture of electrical apparatus. Inpractice, itvhas been quite diflicult to process the known iron-cobaltalloys as they are quite brittle. Where it has been found possible toprocess the alloys, such workable alloys did not have as high anelectrical resistivity as is required of the metal where the metal is tobe employed in alternating current applications with the result thatlosses due to eddy currents were not as low as desired.

One object of this invention is to provide a magnetic alloy of iron,cobalt and chromium.

Another object of this invention is to provide an iron-cobalt alloycontaining a small but effective amount of chromium as an essentialelement thereof and which will have a high saturation.

Other objects of this invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawings, inwhich:

Figure 1 is a graph, the curves of which illustrate the quenchingresponse of the alloys to the carbon content on the elongationcharacteristics of the alloy;

Fig. 21's a graph, the curve of which illustrates the effect ofthickness of the quenched alloy on the elongation, and

Figs. 3, 4, 5 and 6 are graphs the curves of which illustrate the effectof the chromium content on the magnetic and electrical characteristicsof the alloy.

The alloy of this invention comprises from 34.5% to 35.5% cobalt, from0.30% to 0.55% chromium, less than 0.005% carbon and the balancesubstantially all iron. The cobalt content is selected at that value forwhich maximum saturation occurs in the iron-cobalt system together withlow magnetic anistrophy so that the resulting alloy will have the samemagnetic quality in all directions.

As the binary iron-cobalt alloys are quite brittle, such compositionsare modified in accordance with this invention by the inclusion of from0.30% to 0.55% chromium to improve the strength of the alloy and sochange its structure that the resulting alloy can be hot rolled withoutfracture. While chromium within the range indicated makes it possible tohot roll the alloy it is also found that such chromium contents alsoimprove the electrical resistance of the alloy rendering such alloybetter suited for use in electrical apparatus.

Although the iinal carbon content is given as less than 0.005%, inmaking the alloy the carbon content is initially maintained at between0.02% and 0.10% to improve the forgeability characteristics of the alloyand to render it responsive to a quenching treatment as will beexplained more fully hereinafter.

In making the alloy, a small amount of carbon in an amount equivalent to0.1% of the charge to be melted is deposited in the bottom of themelting crucible for deoxidation purposes after which the properproportions of unannealed iron and unannealed cobalt is charged, alprotective atmosphere of hydrogen being employed to pre-` ventoxidation of the iron during the melting. After the charge is molten,the slag is removed and about 0.1% titanium and 0.1% silicon are addedas deoxidizers. Sufficient deoxidizing elements are added to completethe deoxidation of the melt after which from 0.4% to 0.45% chromium andabout 0.05% carbon are added to the melt. Suincient carbon is added toinsure the retention of at least 0.02% carbon in the cast alloy. Whenall of the additions are completely melted the molten alloy is pouredinto a chill mold and an ingot having a composition of 34.5% to 35.5%cobalt, .30% to .55% chromium, 0.2% to .10% carbon and the balancesubstantially all iron is obtained.

In practice, the ingot is hot rolled on a suitable plate mill to a sizeof about 3 thick by 6 wide after which it is cogged at a temperature ofabout 950 C. by conventional practice to a one inch thick sheet bar. Thesheet bar is then heated to a temperature between 900 C. and 1100 C.after which it is passed through reducing rolls to reduce it to a striphaving a thickness between .05 and .10 inch and preferably between .08and .10 inch. v

As the strip leaves the hot rolls it is at red heat and preferably at atemperature between 750 C. and 950 C. The hot strip is passed directlyinto a quenching bath of water or the like to render it ductlle.Referring to Fig. 1 the effect of the carbon content of the alloy on thequenching response is illustrated by curves I0 and |12. Curve lll isillustrative of the iron-cobalt-chromium alloys of this invention whichcontain less than .02% carbon and demonstrates the effect of thequenching temperature on the elongation of such alloys. As illustratedthe elongation is improved by quenching from a relatively narrow rangeof 700 to 760 C. and even then the elongation obtained is only about 10to 12%. Such range is too critical where the alloy is to be quencheddirectly asv it leaves the hot reducing rolls and the ductility thusimparted is unsatisfactory. Y

On the other hand, if the alloy contains over .02% carbon, curve l2illustrates that good elongation, which is a measure of ductility, isobtainable over avi/'ideV range of quenching temperatures of from'750"C. to 950 C. Such temperatures can readily be maintained in the strip asit v leaves the hot rolls and it'is therefore evident that the,quenching response of the alloys containing over .02% carbon is Vofgreat importance from commercial processing consideration as the alloyscan be quenched directly from commercial hot mills. Y

The curves l and l2 are composite curves based on a number of alloyshavingY the ironcobalt-chromium contents given hereinbefore and withcarbon contents above and below .02%V

and which were hot'rolle'd'to athickness of .05 inch and quenched fromthe temperatures indicated.v In all cases the Yquenchingresponse'at'lthe different quenching temperatures formed the `generalpatternillustrate'd by curves lll `and t2 depending upon whether or notthe carbon content was above or' below .02%.

Referring to Fig. 2 curve'lvl4 is la composite curve based on tests ofa'representativevalloy within thev range of elements given hereinbeioreing a thickness of not more than .025 inch and formed to anypredetermined shape in which it is to be employed in industry.. InVpractice the alloy strip can be readily reduced to aV thin sheet havinga thickness of .002 inch. Y

When the strip is worked to size and shape, itV

is then subjected to an annealing treatment'consisting of heating'thesheetffor afperiod'of time at a temperature between875" and 925 C. inanon-carburizing and non-oxidizing" atmosphere such as hydrogen orcracked ammonia. Preferably,V ifthe alloy sheet has a low carboncontentof 'from .02% to .05%, the sheet is annealed-in rcomnfiercialy-dryrhydrogen having a dew point of 30, it being found that With theincidental oxide and' moisture on the alloy sheets and in the K,annealngfurnace that the gaseous atmosphere and illustrates the effectof the thickneesroithe strip on the elongation `when quenched from atemperature of 910 C. Pis illustrated, the elongation decreases astherthi'cknessdecreases it be'- ing undesirable to quench strips ofAless than .05

Yinch as the required ductility. cannot be obtained,

As stated hereinbefore, in accordance with this invention the strip ishot reduced to a thickness of .(15 to .10 inch it' being found. thatsuchsizedstrips containing more 'than .02% carbonjvill have anelongation of 15% or mor e. Preferably the strip is reduced to .l inchor. less in order to facilitate the cold rolling of the'quenched stripto size. Also it is preferred thatthe thickness be not less than L08inch as thepthinner strips tend to buckle Vwhen quenched, being sothinthat theyV areV hard to handle although strips having a thickness. of.05 inchV can be quenched Without attending buckling if care isexercised inhandling such thin strips. Y y While the quenching .of thehot reduced strips containing over .02% carbon fromk between'7-50 and950 C. renders thestrips ducti'le, the reason for imparting ductilitythereto is not apparent y and noreason for such'a characteristic can beadvanced. Apparently the ductility of suchV a quenched strip is.notidependent upon a crystal structure change for observations haveproven that there are Y'no crystal. structure changesunless the strip isheated above 950 C. Againfreferring toFig. 2, lthe improvementinductil-ity imparted by quenching theV hot reduced strip is illustratedbyreference to the point lvwhich indicates the elongation-of oneofthe/alloys subjected tothe hot reducing ,but Vwhich was. no t 'quenchedas compared with theupper righthand end of curve. I4 lwhich representsthe elongation.

of the same hot reduced `alloy striplasV quenched from 910 C. Theimprovement in ductility imparted to the alloy strip by the quenchingtreat- Y ment is outstanding. K

Afterthe alloy strip isfiquenchedto render it ductile :as Vdescribed'hereirrbefora it 'canbe readilyzgcoldiwcrked asfbycoldirollingito vasheethavwill effectively decarburize the alloy. If, however, the carboncontent of the alloyV sheet is high, for example between .05% and .1then it is preferred toanneal the sheets in wet hydrogenhaving up to 3%moisture` by volume to effect" the decarburization of the alloy. In lallcasesit-is desired to so decarburize the alloy during "the anneal thatnot more than '005% carbon "re-.j

mains in the alloy. -Byreducing thecarbonin this'mannerga low lossmaterial is obtained'and grain growth in the alloyzis hastened. i

In annealing the alloy strip,`theV stripv is subjected to the annealingtemperatureY for av period of time of from 10'to 50 hours, thelattertimebeing preferred as such period of time gives the grains'an opportunityto'grow and thereby `produce a magnetic material' having a lowhysteresis value. Inannealing the sheets, it ispreferred to separate thestrips by means of a suitable refractory material such V as magnesia,talc or alumina, in order to prevent sticlingrorwelding of-theadjacentstrips. Y Y

Referring Yto Figs.Y 3, 4, 5 and 6 the curves thereof demonstrate theadvantages of having a chromium content between 30%' and 255%. In Fig. 3curve i6 illustrates the effect o i' the chromium content on thecoercive force: ofthe alloys, the curve being a composite curve ofdif- Yferent alloys containing chromium as indicated.

As 'is' evident the alloys containingbetween 130% and .55% and inparticular between 30% `and .40% chromium have quite lowjjcoerciveiforcewhich isa highly desirable characteristic in Amagnetic alloys of thistype. Y

CurvesA I8 and 270 oi Figures l and 5,..respee tively, represent themagnetizing.. forces fori-1:10

and IIL-10G oersteds forV alloys havingdifierentchromium. contents. Theeiiect of the chromium content is not so critical at the higherlux'densities as represented by curve 2-0'- although-there is adeiinite'decrease in the magnetizing forcek as the chromium content isincreased. However; iatsevereA handling sometimes' being1droppedlbythe".

operator in passing from one step of the processing to another. In orderto further strengthen the alloy Without detrminetally affecting itsmagnetic characteristics, it may be desirable to include from .05% to.20% of metal selected from the group consisting of manganese,molybdenum, tungsten, titanium or silicon in the alloy as it has beenfound that such elements within the range given will improve thestrength of the alloy somewhat. These elements, however, do not aid inenhancing, nor do they detract from, the magnetic properties of thebasic iron, cobalt and chromium alloy and satisfactory magnetic alloysheet can be readily produced either` with or without such minorquantities of strengthening elements.

As examples of some of the thin alloy sheets and their magneticcharacteristics produced in accordance with this invention, referencemay be had to the following table, it being understood that in additionto the cobalt and chromium contents listed the alloys have less than.005% carbon and the balance iron.

chromium less than .005 carbon and the balance iron with .06% molybdenumto strengthen the alloy when reduced to size as described and annealedat 925 C. for 10 hours had a coercive force of 1.35 oersteds but whenthe annealing period was extended to hours, the coercive force decreasedto .61 oersted. When thus annealed, the alloy sheet had a tensilestrength of 59,000 pounds per square inch and an elongation of 3.6%.

In another alloy containing 34.5% cobalt, .41% chromium, less than .005%carbon and the balance iron with less than .20 silicon to strengthen thealloy when reduced to size as described and annealed at 900 C. for 10hours had a coercive force of 0.68 oersted and a permeability for 11:10of 1685.

The alloys of this invention can be readily duplicated, the methoddescribed making it possible to produce thin sheets of the alloy and tofabricate it into the form necessary for utilizing the magnetic alloy asa component in electrical apparatus.

I claim as my invention:

1. A magnetic alloy composed of 34.5% cobalt, 0.30% to 0.55% chromium,less than 0.005% carbon, and the balance substantially all iron.

2. A magnetic alloy composed of about 35% cobalt, about 0.37% chromium,less than 0.005% carbon, and the balance substantially all iron.

JAMES K. STANLEY.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS

